Halogens: Metal halides. Reaction with conc. sulphuric acid. Precipitation reactions with silver and lead ions. Tests for halide ions

Concentrated sulfuric acid, when reacted with metal halides, can act as either an acid or an oxidizing agent.
If it is acting as an acid, the concentrated sulphuric acid gives a hydrogen ion to the halide ion to produce a hydrogen halide.
e.g. NaCl + H2SO4 --> HCl + NaHSO4
If it is acting as an oxidizing agent, how it reacts with halides depends on the halide itself.
Concentrated sulfuric acid is not a strong enough oxidizing agent to oxidize fluoride or chlorine ions.
Bromide ions are strong enough to reduce concentrated sulfuric acid. It will reduce sulfuric acid to sulfur dioxide gas and it will be oxidized to bromine.
Iodide ions are stronger reducing agents than bromide ions, so they will reduce sulfuric acid to sulfur dioxide, then to sulfur, then to hydrogen sulfide. 

You can test for halide ions by using silver nitrate. If silver nitrate solution is added to water containing halide ions the silver halide is precipitated. This is because silver halides are insoluble in water.
The same occurs for lead nitrate; the lead halide is precipitated.




http://www.chemguide.co.uk/inorganic/group7/halideions.html
http://www.bbc.co.uk/schools/gcsebitesize/science/add_edexcel/ionic_compounds/ionicanalysisrev5.shtml
http://www.nuffieldfoundation.org/practical-chemistry/silver-and-lead-halides